1. Field of the Invention
This invention relates to a method and apparatus for liquefying natural gas. In another aspect, the invention concerns an improved liquified natural gas (LNG) facility employing an enhanced nitrogen removal system. In still another aspect, the invention relates to a method and apparatus for removing nitrogen from a relatively warm natural gas stream.
2. Description of the Prior Art
The cryogenic liquefaction of natural gas is routinely practiced as a means of converting natural gas into a more convenient form for transportation and storage. Such liquefaction reduces the volume of the natural gas by about 600-fold and results in a product which can be stored and transported at near atmospheric pressure.
Natural gas is frequently transported by pipeline from the supply source to a distant market. It is desirable to operate the pipeline under a substantially constant and high load factor but often the deliverability or capacity of the pipeline will exceed demand while at other times the demand may exceed the deliverability of the pipeline. In order to shave off the peaks where demand exceeds supply or the valleys when supply exceeds demand, it is desirable to store the excess gas in such a manner that it can be delivered when demand exceeds supply. Such practice allows future demand peaks to be met with material from storage. One practical means for doing this is to convert the gas to a liquefied state for storage and to then vaporize the liquid as demand requires.
The liquefaction of natural gas is of even greater importance when transporting gas from a supply source which is separated by great distances from the candidate market and a pipeline either is not available or is impractical. This is particularly true where transport must be made by ocean-going vessels. Ship transportation in the gaseous state is generally not practical because appreciable pressurization is required to significantly reduce the specific volume of the gas. Such pressurization requires the use of more expensive storage containers.
In order to store and transport natural gas in the liquid state, the natural gas is preferably cooled to −240° F. to −260° F. where the liquefied natural gas (LNG) possesses a near-atmospheric vapor pressure. Numerous systems exist in the prior art for the liquefaction of natural gas in which the gas is liquefied by sequentially passing the gas at an elevated pressure through a plurality of cooling stages whereupon the gas is cooled to successively lower temperatures until the liquefaction temperature is reached. Cooling is generally accomplished by indirect heat exchange with one or more refrigerants such as propane, propylene, ethane, ethylene, methane, nitrogen, carbon dioxide, or combinations of the preceding refrigerants (e.g., mixed refrigerant systems). A liquefaction methodology which is particularly applicable to the current invention employs an open methane cycle for the final refrigeration cycle wherein a pressurized LNG-bearing stream is flashed and the flash vapors (i.e., the flash gas stream(s)) are subsequently employed as cooling agents, recompressed, cooled, combined with the processed natural gas feed stream and liquefied thereby producing the pressurized LNG-bearing stream.
Natural gas streams frequently contain relatively high concentrations of nitrogen. High nitrogen concentrations in natural gas that is subjected to liquefaction in a LNG facility may present one or more of the following drawbacks: (1) the natural gas can be more difficult to condense; (2) the heating value of the natural gas used as fuel gas for the LNG facility's gas turbines can be greatly diminished; and (3) LNG produced by the facility may be out of spec. Thus, many LNG facilities employ nitrogen removal units (NRUs) to lower the concentration of nitrogen in the natural gas stream to an acceptable level. In the past, these NRUs have typically required significant chilling of the NRU feed stream in order to provide effective nitrogen removal.
The requirement that the feed stream to conventional NRUs be significantly chilled has the disadvantage of increasing the total installed cost of the LNG facility. In many conventional LNG facilities, the feed stream to the NRU must be withdrawn from a “cold box” and the reduced-nitrogen product stream from the NRU must be reintroduced into the “cold box.” A “cold box” is simply an insulated enclosure that houses a certain group of low-temperature components of a LNG facility. Cold boxes are used because they are less expensive and more efficient than individually insulating each low-temperature component. However, those skilled in the art recognize that each penetration into and out of a cold box complicates the design of the cold box, thereby adding to its cost. In addition, the flow lines between the cold box and NRU of a conventional LNG facility require insulation due to the low temperature of the stream flowing therethrough. Obviously, insulated lines are more expensive to install and maintain than non-insulated lines.